Clinical Scenario:
A 10-year-old boy presents with recurrent episodes of severe left flank pain for the past year. The episodes are occasionally associated with vomiting, and he has passed small urinary stones on two occasions. There is no history of recurrent urinary tract infection.
Urinalysis shows microscopic haematuria. Renal ultrasonography demonstrates a left renal calculus. Analysis of the stone shows a high concentration of cystine.
Which renal tubular abnormality best explains this child’s condition?
A. Defective citrate reabsorption
B. Defective cystine reabsorption
C. Excess uric acid excretion
D. Impaired phosphate reabsorption
E. Increased oxalate absorption
Correct answer & Explanation:
Correct answer: B. Defective cystine reabsorption
Explanation
The child has cystinuria, an inherited disorder of proximal tubular amino-acid transport. There is impaired renal reabsorption of cystine and the dibasic amino acids ornithine, lysine and arginine (COLA).
Because cystine has relatively low urinary solubility, particularly in acidic urine, it can precipitate and form recurrent cystine stones, often beginning during childhood or adolescence.
The main transport defects involve the SLC3A1 and SLC7A9 genes, which encode components of the proximal tubular transporter responsible for reabsorption of these amino acids.
Why the other options are wrong
- A. Defective citrate reabsorption: would predispose to hypocitraturia and calcium stones, not cystine stones.
- C. Excess uric acid excretion: causes uric acid nephrolithiasis.
- D. Impaired phosphate reabsorption: causes renal phosphate wasting rather than cystine nephrolithiasis.
- E. Increased oxalate absorption: predisposes to calcium oxalate stones.
Exam pearl
Child with recurrent nephrolithiasis + cystine stone → cystinuria → defective proximal tubular reabsorption of cystine + COLA.
Classic urine microscopy clue: hexagonal cystine crystals.
Reference:
- Eggermann T, et al. Cystinuria: an update on pathogenesis, diagnosis and treatment. Pediatric Nephrology.
- Sinha A, et al. Cystinuria and cystine stones. StatPearls, NCBI Bookshelf. Cystinuria — StatPearls, NCBI Bookshelf
